The present invention relates to a process for the preparation of alpha mercuric iodide monocrystals (.alpha.-HgI.sub.2).
Hitherto, various methods have been used to attempt to attain alpha mercuric iodide monocrystals having an adequate crystalline quality for purity. These methods have in particular consisted of growth in solution and growths in the vapour phase. On the basis of the vapour phase, a known process consists of growing a .alpha.-HgI.sub.2 monocrystal at the end of a sealed quartz tube by mass transfer in vacuo from an annular .alpha.-HgI.sub.2 polycrystalline source placed in the other end of the tube. The temperature of the crystal is kept constant, for example at 100.degree. C., whilst the temperature of the source varies between 1.degree. and 2.degree. C. and 98.degree. C.
This process makes it possible to obtain .alpha.-HgI.sub.2 monocrystals of up to 30 g within 3 to 12 days. On the basis of crystals obtained according to this process, high-performance X and .gamma.-detectors have been produced having a product .mu..sub.e .tau..sub.e of the order of 10.sup.-4 cm.sup.2 /V and a product .mu..sub.h .tau..sub.h of the order of 10.sup.-6 cm.sup.2 /V. In this case, .tau. designates the mobility of the charge carriers and .mu. the life for electrons with index e and for holes with index h. However, this process has a number of disadvantages.
The main disadvantage is linked with its operating principle in which growth periods follow etching periods. As a result, the crystal is marked by growth rings where are concentrated growth defects and impurities.
Another major disadvantage is the stoichiometry of the crystal which cannot be controlled during growth. Crystals growing from high purity polycrystals only grow with difficulty having a prismatic-pinacoid habit and being rich in mercury. The crystals develop by advancement of the faces (110) of the prism which easily trap impurities (mercury in excess counts as an impurity) and growth defects are propagated throughout the crystal mass. The addition of iodine during the growth blocks the faces which can be no longer making the monocrystal grow.
Moreover, the crystal grows from a nucleus obtained by spontaneous nucleation, i.e. at the start of growth the nucleus which is to be grown is selected from a number of nuclei. However, in the case of spontaneous nucleation, it is not possible to control the orientation of the crystal relative to the tube and as the nucleus is never perfect, it still produces defects which are propagated in the crystal mass. At the end of growth, the crystal obtained is cooled to ambient temperature, so that it undergoes supplementary deformations.
Thus, this preparation process does not make it possible to systematically obtain satisfactory monocrystals. It is not reproducible and the crystals are never completely usable. After growth, it is necessary to cut off the good parts, which only represent 3/4 of the crystal volume. The density of the dislocations is approximately 10.sup.4 -10.sup.7 cm.sup.-2.
An .alpha.-HgI.sub.2 purification process is also known involving fusion of the zones and successive sublimations. This process makes it possible to obtain high purity .alpha.-HgI.sub.2, but it is extremely laborious to perform and gives a relatively low yield. Thus, to achieve high purity, 30 to 100 fusions of zones and 6 to 30 successive sublimations are necessary. Only one process gives satisfactory results and this forms the subject matter of French Application No. 76 02 031 filed on Jan. 26th 1976 by the Commissariat a l'-Energie Atomique.
This known process for the preparation of alpha mercuric iodide monocrystals consists of dissolving alpha mercuric iodide and at least one dialkyl sulphoxide in an organic solvent, the molar ratio of the dialkyl sulphoxide to the solvent being between 0.01 and 0.1 mol of dialkyl sulphoxide per mol of solvent and the molar ratio of alpha mercuric iodide to dialkyl sulphoxide being between 0.5 and 0.6 mol of alpha mercuric iodide per mol of dialkyl sulphoxide. Alpha mercuric iodide monocrystals are then grown from this solution.
The dialkyl sulphoxides used are selected from the group including dimethyl sulphoxide, methyl ethyl sulphoxide and diethyl sulphoxide, preference being given to dimethyl sulphoxide (DMSO).
The organic solvents used are moderately polar saturated aliphatic solvents which are stable in the presence of mercuric iodide and in particular esters resulting from the esterification of acetic acid by saturated aliphatic alcohols, such as methyl acetate, ethyl acetate, propyl acetate and butyl acetate, saturated aliphatic monoketones such as acetone, methyl ethyl ketone, diethyl ketone and dipropyl ketone and saturated aliphatic nitro derivatives such as nitroethane and nitropropane. The alpha mercuric iodide monocrystals are grown by lowering the temperature of the solution, by evaporating the solvent or by circulating the solution between a container containing the mercuric iodide polycrystals and a container containing monocrystalline nuclei of alpha mercuric iodide. The alpha mercuric iodide forming the starting substance is synthesised from 5 N iodine and 7 N mercury dissolved in a mixture of dimethyl sulphoxide and ethyl acetate and further purified by successive recrystallization processes using solutions of the same composition.
Although this process is highly satisfactory it still has three disadvantages. Firstly, the alpha mercuric iodide monocrystals obtained have a significant deviation compared with the exact stoichiometric composition due to mercury enrichment. The second disadvantage is that alpha mercuric iodide monocrystals are obtained with a prismatic-pinacoid habit, so that the crystals obtained have certain imperfections due to edge dislocations and stack defects in the growth sectors corresponding to the advance of the faces (110). The third disadvantage results from the fact that alpha mercuric iodide solutions become impure during synthesis, purification and growth due to the dissolving of impurities from the borosilicate glass used for all the containers receiving the solutions.
All these imperfections linked with the deviation relative to the stoichiometry, the dislocations and the impurities are disadvantageous for certain applications of alpha mercuric iodide monocrystals and in particular their use in detectors and X and gamma radiation spectrometers.
The process according to the invention obviates the disadvantages referred to hereinbefore and in particular makes it possible to obtain alpha mercuric iodide monocrystals having an octahedralpinacoid habit, thus eliminating the edge and/or screw dislocations and the stacking defects in the nucleus due to the advance of the octahedral faces (011).
Another process for the preparation of alpha mercuric iodide monocrystals is known in U.S. Pat. No. 3,969,182 which uses DMSO as the solvent or more correctly as the complexing agent. In this process, HgI.sub.2 is dissolved in DMSO at saturation between 20.degree. and 80.degree. C., for example in accordance with the molar ratio of HgI.sub.2 :DMSO of approximately 0.5, the solution is filtered on a fritted glass filter and then the glass container containing the solution is left open in contact with moist air with a 15 to 50% humidity level in order to crystallize for 0.5 to 24 hours. The moisture reduces the solubility of HgI.sub.2 which crystallizes by spontaneous nucleation on the bottom of the container. Crystals of 10.sup.-2 are obtained after 7.5 cm.sup.3. It is also possible to dilute DMSO with xylene or toluene until a saturated HgI.sub.2 solution is obtained with a molar ratio of HgI.sub.2 :DMSO of 0.33, followed by the crystallization of alpha-HgI.sub.2 by moisture absorption in the manner described hereinbefore. This process is unsuitable for practical requirements. The crystals develop faces of the prism (110) and are very distorted. As a result of their crystalline quality and impurities the monocrystals obtained are not usable for nuclear detection. They are characterised by a product .mu..sub.e .tau..sub.e =4.times.10.sup.-8 cm.sup.2 /V and .mu..sub.h .tau..sub.h which is not measurable (.mu. being the mobility of the charge carriers and .tau. their life, as hereinbefore). Thus, it is not possible to obtain monocrystals of .alpha.-HgI.sub.2 without inclusions of DMSO and water from HgI.sub.2 saturated solutions in purity DMSO and the inclusions are highly prejudicial to the quality of the crystals because they make the product .mu..tau. of the electrons (.mu..sub.e .tau..sub.e) low and that of the holes (.mu..sub.h .tau..sub.h) extremely low.
The dilution of DMSO by xylene or toluene makes the HgI.sub.2 monocrystals increasingly unusable as HgI.sub.2 reacts with the aromatic solvents. Organo-metallic compounds are produced which are added to the impurities absorbed by the solution from the glass of the container and in particular fritted glass at the time of filtering.
For all these reasons, the crystals obtained by this process have the disadvantage of not being stoichiometric, mercury being in excess, and they contain a very high level of edge and screw dislocations of approximately 10.sup.4 -10.sup.5 cm.sup.-2.